The present invention relates to a catalyst for the decomposition of N2O, a process for its preparation and a process for the decomposition of N2O using this catalyst.
N2O forms as a byproduct in many processes in which HNO3 is used in the liquid phase as an oxidizing agent. Particularly in the conversion of alcohols, aldehydes and ketones, e.g. cyclohexanol and cyclohexanone, to adipic acid, acetaldehyde to glyoxal or glyoxal to glyoxylic acid, considerable amounts of N2O are liberated. Furthermore, N2O is emitted in the preparation of nicotinic acid and hydroxylamine. N2O also forms the byproduct in the preparation of nitric acid by combustion of NH3.
In an article published in 1991 in Science, 251 (1991), 932, Thiemens and Trogler show that N2O potentially harms the earth""s atmosphere to a certain degree. In the stratosphere, N2O is considered to be an important source of NO, which in turn is said to have a substantial effect on the degradation of ozone in the stratosphere. In addition, N2O is considered a greenhouse gas, and the potential of N2O for heating up the earth is 290 times greater than that of CO2.
Recent years have seen a large number of publications which are concerned with reducing the N2O emissions caused by anthropogenic activities.
The use of catalysts in the reduction or decomposition of N2O makes it possible to carry out the reaction at a temperature level substantially lower in comparison with the purely thermal decomposition.
EP-A 0 687 499 describes a catalyst for catalytic reduction of NOx and/or for oxidation of hydrocarbons in off-gases, which consists of a copper oxide-zinc oxide-aluminum oxide spinel of the chemical formula CuAZncAlDO4, where A+C +D=3, A greater than 0, C greater than 0 and D greater than 0. In this publication, the ratio of Cu and Zn to Al can be freely chosen within wide limits. In an example, NO is reacted with propene of the disclosed catalysts to give N2 and H2O. The decomposition of N2O at high temperatures is not discussed.
WO 94/16798 describes a process for the catalytic decomposition of pure N2O or N2O contained in gas mixtures. The catalyst used is an MxAl2O4 catalyst. This is prepared by mixing CuAl2O4 with Sn, Pb or an element of the 2nd main group or subgroup of the Periodic Table of the Elements as an oxide or salt or in elemental form and then calcining at from 300xc2x0 C. to 1300xc2x0 C. and from 0.1 to 200 bar. According to Comparative Examples 1 and 2, as presented further below in the present description at the beginning of xe2x80x9cExamplesxe2x80x9d, the x value in the formula MxAl2O4 is 0.61 (Comparative Example 1) and 0.76 (Comparative Example 2).
The catalyst systems known from the prior art are suitable for the decomposition of N2O. Their thermal stability at above 500xc2x0 C. is, however, not optimum. One problem which still exists in many cases is the deactivation of the catalysts, necessitating frequent replacement of the catalyst bed Particularly at above 500xc2x0 C., as is advantageous for virtually complete degradation of the N2O with an acceptable amount of catalyst, pronounced, irreversible deactivation occurs.
It is an object of the present invention to provide a catalyst for the decomposition of N2O, which catalyst is thermally stable at high temperatures.
We have found this object is achieved by a copper-containing catalyst for the decomposition of N2O, containing a compound of the formula MxAl2O4, where M is Cu or a mixture of Cu with Zn and/or Mg. According to the invention, x is typically from 0.8 to 1.5 in the catalyst.
The novel catalysts are preferably essentially spinels which may still contain small amounts of free oxides in crystalline form, such as MO (where M is, for example, Cu, Zn or Mg) and M2O3 (where M is, for example, Al). The presence of a spinet phase can be detected by recording XRD spectra. The amount of the oxides in the catalyst is in general from 0 to 5, preferably from 0 to 3.5, % by weight.
The amount of Cu and any Zn and/or Mg should be chosen such that a filled or virtually filled spinet is obtained. This means x in the formula MxAl2O4 is from 0.8 to 1.5, preferably from 0.9 to 1.2, particularly preferably from 0.95 to 1.1. We have found that, for x values below 0.8, the thermal stability is substantially lost. x values above 1.5 likewise lead to a deterioration in the catalyst activity and catalyst stability. The novel catalyst having an x value of from 0.8 to 1.5, preferably from 0.9 to 1.2, particularly preferably from 0.95 to 1.1, in the formula MxAl2O4 is thus a high temperature-stable catalyst for decomposition of N2O. The catalyst has advantageous aging behavior, i.e. the catalyst remains active for a long time without being thermally deactivated.
The novel catalysts contain copper in oxide form, calculated as a copper oxide, CuO, in an amount of in general from 1 to 54, preferably from 5 to 40, particularly preferably from 10 to 30, % by weight, based on the total catalyst.
The novel catalyst may additionally contain further dopants, in particular Zr and/or La, in oxide form. Doping with Zr and/or La further increases thermal stability of the catalysts, but the initial activity is slightly reduced. It is particularly advantageous to introduce Zr and/or La dopants via corresponding element-doped aluminum oxides. The content of the dopant compounds in the novel catalyst is in general from 0.01 to 5.0, preferably from 0.05 to 2, % by weight.
In addition, the novel catalyst may contain further metallic active components. Such metallic active components are preferably metals of the 8th subgroup of the Periodic Table of the Elements, particularly preferably Pd, Pt, Ru or Rh. As a result, it is possible to obtain catalysts which not only are very active at high temperatures but have a very high activity at temperatures as low as below 400xc2x0 C. The novel catalysts can therefore be used in a wide temperature range, which is a major advantage in the case of adiabatically operated N2O decomposition processes. The amount of the metals of the 8th subgroup in the novel catalyst is in general from 0.01 to 5, preferably from 0.1 to 2, % by weight.
The novel supported catalysts may be present in the form of pellets, honeycombs, rings, chips or solid or hollow extrudates or in other geometric shapes. For specific applications, it is important that the shape and size are chosen such that a very small pressure loss results.
The novel catalysts generally have a BET surface area of from 30 to 150, preferably from 50 to 100, m2/g.
The novel catalysts preferably have a bimodal or trimodal pore structure. They contain mesopores of from 10 to 100 nm, preferably from 10 to 30 nm, and macropores of from 100 to 5000 nm, preferably from 100 to 2000 nm. Such catalysts are substantially more active than catalysts having a monomodal pore structure.
The porosity of the carrier should advantageously be such that the pore volume is from 0.10 to 0.70 ml/g.
The novel catalysts can be prepared from oxide starting materials or from starting materials which are converted into the oxide form during the final calcination. They can be prepared by a process in which the starting materials, containing Al, Cu and, if required, Zn and/or Mg, and, if required, further additives, are mixed, converted into moldings and, if required, treated at above 500xc2x0 C. in one step.
In a preferred embodiment of the process a mixture of the starting materials is processed, for example by drying and pelleting, to give corresponding moldings. These are then heated at from 500 to 1000xc2x0 C. for from 0.1 to 10 hours (calcination). Alternatively, a molding material can be prepared with the addition of water in a kneader or mix-muller and extruded to give corresponding moldings. The moist moldings are dried and then calcined as described.
Particularly preferably, the novel catalysts are prepared by a process which comprises the following steps:
a) preparation of a Cuxe2x80x94Al oxide molding,
b) impregnation of the molding with soluble Cu compounds and, if required, Mg compounds and/or Zn compounds,
c) subsequent drying and calcination.
In this process, preferably a carrier is first prepared from Cu in the form of Cu(NO3)2 and/or CuO and an Al component. In the preparation of the carrier, starting materials can be mixed, for example, in dry form or with the addition of water. Zn and/or Mg component(s) can be applied to the carrier by impregnating once or several times. The novel catalysts are obtained by drying and calcination at from 500 to 1000xc2x0 C., preferably from 600 to 850xc2x0 C.
Preferably, Cu is used as a mixture of CuO and Cu(NO3)2. The catalysts thus prepared have a higher mechanical stability than the catalysts prepared only from CuO or only from Cu(NO3)2. It is also preferable to use, if required, corresponding mixtures of oxides and nitrates of Zn and/or Mg. Instead of oxides and nitrates, it is also possible to use pure oxides if acidic molding assistants, such as formic acid or oxalic acid, are also added. Particularly when the novel catalysts are prepared in one step, in which all starting materials are mixed and are further processed to give moldings, it is very advantageous to use mixtures of oxides and nitrates.
Furthermore, it is preferable to use a mixture of Al2O3 and AlOOH as the aluminum component (for example from Condea). Suitable aluminum components are described in EP-A-0 652 805. If, for example, AlOOH and Al2O3 from Condea are used in the ratio of 70% by weight to 30% by weight, catalysts which have a bimodal pore structure are obtained. They are substantially more active than catalysts having a monomodal pore structure.
To increase the activity at relatively low temperatures, in particular at below 400xc2x0 C., metals of the 8th subgroup of the Periodic Table of the Elements, such as Pd, Pt, Ru and Rh, can be applied to the catalysts. Preferably, these noble metals are applied by means of an impregnation step, in the form of their nitrates. The impregnation is followed by decomposition at from 200 to 600xc2x0 C. and reduction to the elemental noble metal. Other, known processes may also be used for applying the noble metals.
The novel catalysts are suitable for the decomposition of N2O. The present invention therefore furthermore relates to a process for the catalytic decomposition of pure N2O or N2O contained in gas mixtures, at elevated temperatures, in which a catalyst according to the present invention is used.
Preferably, the novel catalysts are used for the decomposition of N2O in N2O-containing off-gas streams, as obtained, for example, in processes for the preparation of adipic acid, nitric acid, hydroxyl amine derivatives, caprolactam, glyoxal, methylglyoxal, or glyoxylic acid or in processes for the combustion of nitrogen-containing materials, e.g. NH3.
The process for the decomposition of N2O in off-gases from the preparation of adipic acid and from the preparation of nitric acid is particularly suitable. The novel process for purifying process gases from ammonia combustion is firthermore suitable.
The N2O can be eliminated from the nitric acid off-gases without decomposing further oxides of nitrogen, NOx, (desired products) in significant amounts. Further oxides of nitrogen are nitric oxide (NO), dinitrogen trioxide (N2O3), nitrogen dioxide (NO2), dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N2O5) and nitrogen peroxide (NO3). The content of oxides of nitrogen, NOx, may as a rule be from 0 to 50, preferably from 1 to 40, particularly preferably from 10 to 30, % by volume, based on the total gas.
The process is suitable for purifying off-gases whose N2O content is from 0.01 to 50, preferably from 0.01 to 30, particularly preferably from 0.01 to 15, % by volume, based on the total gas.
In addition to N2O and further oxides of nitrogen, NOx, the off-gases may also contain, for example, N2, O, CO, CO2, H2O and/or noble gases, without this substantially affecting the activity of the catalysts. Slight inhibitions of the catalyst activity can be compensated by increasing the catalyst volume or by reducing the loading.
Owing to the high thermal stability of the novel catalysts, the novel process can be carried out at up to 1100xc2x0 C. In general, the process can be carried out at from 200 to 1100xc2x0 C., preferably from 450 to 1000xc2x0 C., particularly preferably from 500 to 900xc2x0 C. The high thermal stability of the novel catalysts permits load change without problems. The thermal deactivation of the novel catalysts at high temperatures is substantially smaller than that of the catalysts known from the prior art, as shown by the following examples. The La- and Zr-doped catalysts are particularly stable to aging. If the novel process is carried out at relatively low temperatures (from 200 to 500xc2x0 C.), doping the novel catalysts with noble metals is advantageous.